A shortcoming of global positioning system (GPS) based navigation and steering control systems used in agricultural ground vehicles is that the global positioning system receiver of such systems can only determine the position of the global positioning system antenna. On most ground vehicles, the mounting location for the global positioning system antenna is constrained by the requirement that a clear view of the sky, and thus the global positioning system satellites, be provided to the antenna.
When a work vehicle, like an agricultural tractor, pulls an implement over a field, and an automatic steering system is provided, the antenna of the location signal generation arrangement for providing location data of the actual position to the steering system is usually located on the vehicle. The vehicle is automatically steered in a manner such that the implement is moved on an intended path. The offset between the antenna and the implement can be compensated for by algorithms taking the geometry and kinematics of the tractor/implement combination into account. In this context, reference is made to U.S. Pat. No. 6,434,462.
If only the intended path of the implement and the actual position of the towing vehicle are considered for calculating steering signals for a steering actuator of the vehicle, and the implement and vehicle are moving on laterally inclined ground, possible deviations of the implement from the intended path may be caused by the lateral forces pulling the implement downhill.
It has been proposed to sense the lateral inclination and control steerable wheels of the implement to automatically compensate for the lateral inclination (U.S. Pat. No. 7,147,241), or to control a laterally movable hitch of a tractor dependent on a sensed lateral slope inclination, such that the implement stays on the intended path even on laterally sloped surfaces (U.S. Pat. No. 7,162,348). In both proposals, the tractor is provided with a GPS antenna and steered automatically or by an operator based upon a display in a manner such that the tractor remains on the intended path. These solutions require additional actuators for the steerable wheels of the implement or the movable hitch.
Further, it has been proposed to detect the lateral inclination of a tractor to apply lateral offset compensation to the location data based upon the signal from the inclination sensor (U.S. Pat. Nos. 6,501,422 and 6,694,260, US Patent Publication No. 2004/0186644, and European Patent Application EP 1 444 879). This lateral offset compensation takes the lateral movement of the antenna of the location signal generation arrangement with respect to the ground below the vehicle due to the lateral slope into account, but not the gravity forces moving the implement downhill, and can thus not compensate the latter.
Finally, it has been proposed to sense the lateral inclination of the vehicle and to consider the inclination in a determination of steering signals for automatic steering to keep the vehicle on an intended path on a lateral sloped terrain (European Patent Application 1 688 027, and U.S. Pat. No. 3,946,825), with the inclination value being derived from a topographical database (US Patent Publication No. 2005/0096802). These prior art documents only compensate for forces acting on the vehicle, but not on a towed implement.
Thus, there is a need for a reliable system for automatically steering a combination of a work vehicle and a towed implement compensating gravity forces acting onto the implement on lateral slopes.
It is an object of the present invention to provide a simple and reliable system for automatically steering a combination of a work vehicle and a towed implement that is able to steer the tractor in manner such that the implement keeps on the desired path even on terrain with lateral slopes.